Add PWA support and Google Takeout subscription import

- manifest.json + icon.svg for installability on mobile (standalone mode)
- index.html: theme-color, apple-mobile-web-app meta tags, manifest link
- Settings: Import CSV section reads Google Takeout subscriptions.csv,
  extracts UC... channel IDs, calls follow-bulk to follow them all at once

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/public/icon.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <rect width="512" height="512" rx="96" fill="#09090b"/>
+  <text x="256" y="340" font-family="Arial, sans-serif" font-weight="900" font-size="240" text-anchor="middle" fill="#facc15">YT</text>
+</svg>